In the field of projection printing and especially in the semiconductor industry and in the manufacture of integrated circuits the use of pellicles to shield a photomask from airborne particulate matter has gained increasing acceptance and use because of the multitude of benefits derived from their use. The employment of a pellicle for projection printing is described, for example, in U.S. Pat. No. 4,131,363, issued Dec. 26, 1978 to V. Shea et al. and assigned to International Business Machines Corp. The use of such pellicles in projecting printing has resulted in a reduction in mask and printing defects, increased mask reticle life, reduced aligner downtime, increased mask quality and increased device and reticle yield. Materials employed in the prior art and suitable for the production of films utilizable as pellicle films include polymer films of polyoxyethylene terephthalate, nitrocellulose and parylene.
The prior art polymer films while finding usefulness in projection printing systems as pellicle films have been severely limited in their usefulness. For example, while pellicles of such polymer films are useful in the near UV region (wavelengths 340-436 nm) and some usefulness in the mid UV region (wavelengths 280-340 nm) none has been acceptable for use in the deep UV region (wavelength 200-280 nm). Prior art pellicle films, while providing acceptable properties for use in the near and mid UV regions, have not provided acceptable light transmission properties in the deep UV region and thereby cannot be utilized in the deep UV region.
Furthermore, due to the advancing state of the art in this projection printing field and in view of image geometries of less than 2 microns and device packing density approaching the VLSI and VHSI it is highly advantageous that projection imaging wavelength in the deep UV region be employed. Heretofore it has not been possible to enjoy the benefits derived from the use of pellicles in this deep UV region because of the unavailability of any pellicle acceptable for use in the deep UV region.
Additionally, although the prior art pellicle films have found usefulness in the near and mid UV regions, their performance has not been entirely satisfactory. Among the many factors detracting from their usefulness in these regions is the fact that their light transmission properties are not substantially 100 percent at the projection wavelengths but rather are generally 98% or less. Also, film tensile strength of the pellicle films has generally been in the range of 3,000-5,000 psi and the need for a film of increased tensile strength to increase the pellicle film life is desirable. Additionally, the percent of undesirable light scattering in said prior art pellicle films has generally been in the range of 0.3% at 365 nm. A film of decreased light scattering is highly desirable. Also with the prior art pellicle films, the percent etaloning, namely the difference between peak and minimum transmission, has only been as low as about 18% and it is of great advantage to reduce such etaloning as much as possible to increase the light transmission properties of the pellicles. Moreover, the prior art pellicle films, while desiring to obtain the best uniformity of film thickness so as to decrease exposure variations through the pellicle, have generally only obtained films with thickness variations of about 500 .ANG./cm. It is certainly preferred to obtain an otherwise acceptable film which has a much lower variation in film thickness. Prior art films have also suffered greatly from undesirable film defects such as haze, imbedded particles, pinholes, film gels, speckles, striations and other optical defects, which have adversely affected the overall performance of the pellicle films.
It is therefore highly desirable that a pellicle film be available which is useful in the deep UV region and which is as good or better in performance characteristics and/or physical properties as has been the case with pellicle films used in the mid or near UV regions. Moreover, it is also highly desirable that such pellicle film be useful in all three UV regions and that the performance characteristics and/or physical properties of such pellicle film in the mid and near UV regions be improved in whole or in part compared to the performance characteristics and/or physical properties of the materials heretofore employed in the prior art for pellicle films in the mid or near UV regions.